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Fabricating high-density microarrays for retinal recording

Mathieson, K and Cunningham, W and Marchal, J and Melone, J and Horn, M and O'Shea, V and Smith, KM and Litke, A and Chichilnisky, EJ and Rahman, M (2003) Fabricating high-density microarrays for retinal recording. Microelectronic Engineering, 67-8. pp. 520-527. ISSN 0167-9317

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Abstract

Understanding how the retina encodes the visual scene is a problem, which requires large area, high-density microelectrode arrays to solve. The correlated signals that emerge from the output (ganglion) cells of the retina form a code, which is not well understood. We use a combination of electron beam lithography, photolithography and dry-etch pattern transfer to realise a 519-electrode array in the transparent conductor indium tin oxide (ITO). The electrodes are spaced at 60 μm in a hexagonal close-packed geometry. A mix and match lithography procedure is utilised, whereby the high-density inner region is fabricated using electron beam lithography whilst the outer sections are realised by photolithography. Reactive ion etching (RIE), using CH4/H2, of the ITO forms the array structure and SF6 RIE allows resist removal and patterning of vias through a plasma deposited Si3N4 protective layer. The electrical properties of the ITO layer are unaffected by the etching procedures. A reliable method for achieving low-impedance electroplated platinum electrodes has been employed to yield electrode impedances of ∼20 kΩ. An array fabricated using these dry-etch techniques is shown to record action potentials from live retinal tissue in neurophysiological experiments.